This invention relates to apparatus for monitoring the operation of a sheet transport mechanism, but more particularly to apparatus for detecting the presence of an image receiving medium or transfer member, e.g., paper, within the cleaner brush housing of an electrostatic reproduction machine.
In the practice of xerography as described in U.S. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to selectively dissipate a charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with a configuration of the original light pattern.
The electrostatic latent image may then be developed by contacting it with a finely divided electrostatically attractable material, such as a resinous powder. The powder is held in the image areas by the electrostatic field on the layer. Where the field is greatest, the greatest amount of material is deposited; and where the field is least, little or no material is deposited. Thus, a viewable powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper (the example used hereafter) or other image receiving medium and suitably fused to thereby form a permanent print.
It is contemplated that the developed viewable image which is supported on the support surface is to be dynamically transferred to the paper. Thus, the support surface is capable of being continuously transported in synchronism with the paper that is transported to a transfer station. At the transfer station, the paper contacts the support surface in a manner to facilitate the transfer of the developed image to the paper. Subsequent to the transfer of the developed image, the paper is removed from the support surface and transported to subsequent operating stations, such as a fusing station. Concurrently therewith, the support surface is transported to further operating stations where the support surface is cleaned and prepared for subsequent reuse thereof.
Although the foregoing is a general description of the normal operation of a portion of a typical electrostatic reproduction machine, it is not unreasonable to expect that, at times, the paper to which the developed image has been transferred might not be removed from the support surface subsequent to the transfer operation. In this instance, a final copy of the reproduced image will not be received and, moreover, the support surface cannot be properly reused for subsequent reproducing operations. Furthermore, the presence of the paper on the support surface at stations other than the transfer station disrupts the proper operation of the electrostatic reproduction machine resulting in the possibility of serious damage to the machine. Various prior art electrostatic reproduction machines have attempted to provide against these undesired occurrences by monitoring the progress of an image receiving medium as it is transported through the various operating stations of the machine. Accordingly, if a sheet of paper is detected at a location along the transport path prior to the transfer station, but not at a location along the transport path following the transfer station, it is presumed that the paper has not been removed from its contacting relationship with the support surface and further operation of the electrostatic reproduction machine is terminated.
One disadvantage of prior art reproduction machines is the failure of the jam detection mechanism to safeguard the operation of the machine if superposed or multiple sheets of paper are transported to the transfer station. More particularly, if superposed sheets are transported into a contacting relationship with the support surface, the developed image will be transferred to the lower sheet and the upper sheet will be successfully conveyed to the further operator stations, such as the fuser station, by the sheet transport mechanism. The lower sheet, however, will remain in contact with the support surface, and may enter the cleaner-brush housing, which may result in serious damage of the machine. Nevertheless, the presence of a sheet of paper will be detected at a location along the transport path following the transfer station. Consequently, the electrostatic reproduction machine will continue to operate notwithstanding the occurence of a condition that requires correction.
Also, some systems indicate that there is a paper jam, but these systems do not always indicate the location(s) of the jam. In such instances, the jam may be located in several areas including the cleaner-brush housing. If the operator is not aware that paper is in the cleaner-brush housing, he may clear the jam in the other location, and restart the machine. If the paper in the cleaner-brush housing remains undetected for a period of time, the cleaning mechanism will not function effectively, thus eventually resulting in large accumulations of toner dust within the machine, poor quality, and possible breakdown of the machine.